Fossilization

 Fossilization is the process through which a fossil is formed. There are many different types of fossilization, including petrification, carbonization, unaltered preservation, recrystalization, etc.

Replacement

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One of the most common forms of fossilization is replacement. The steps of replacement are as follows:
1. An organism dies and its bones are left in an unaltered state.
2. Over time, winds blow sand and sediments over the remains. Eventually, they are buried deep underground, where they are undisturbed by living creatures.
3.Minerals in the bones are gradually replaced with minerals from the soil and sand, turning the bone to rock.
(picture from: fossilmuseum.net)